1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle.
This application is based on Japanese Patent Application No. Hei 11-26712, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In recent years, electric vehicles and hybrid vehicles have been developed in order to reduce the discharge of carbon dioxide, in consideration of the global warming effect. These vehicles have a motor which does not discharge any gas, and a battery for supplying electric energy.
To charge the battery, external chargers are used for the electric vehicles, while on-vehicle generators are used for the hybrid vehicles.
In parallel hybrid vehicles, driving motors act as the on-vehicle generators, while series hybrid vehicles employ generators rotated by the engines. In addition, hybrid vehicles which are chargeable by an external charger have been developed.
In the parallel hybrid vehicle, the motor assists the driving force from the engine when the vehicle accelerates, while the motor acts as a generator for regenerating electric power for the battery when the vehicle decelerates. The motor repeats the driving and regeneration, that is, the battery repeats charging and discharging. As the result, the battery may not be always fully charged. Because of the repetition of charging and discharging, the remaining charges in the cells in the battery may become unequal, and this inequality may degrade the performance of the battery. Therefore, it is necessary to equalize (refresh) the remaining charges in the cells in the battery.
In the conventional technique, the charges in the cells can be equalized by connecting an external charger to the vehicle and fully charging the battery.
To cope with the next assist or regeneration, the remaining charge in the battery is usually maintained within the medium range. When the vehicle continuously accelerates so that the motor continuously assists the engine, the remaining charge may be gradually decreased, and the battery must be charged later. An external charger can be used for this purpose.
In addition, when using the battery, it is necessary to know the full charge of the battery. Since the hybrid vehicle that is chargeable by the external charger is started after the battery was fully charged, it is inconvenient that the driver cannot know the full charge. Further, to charge the battery by power regenerated from the driving motor or by the rotation of the engine, it is important to detect the full charge state in order to calculate the charge remaining in the battery.
There are two conventional methods for detecting the full charge state.
One method detects the full charge based on the decrease in the voltage, utilizing the phenomenon that the voltage (-.DELTA.V) from the battery decreases when the battery is nearing the full charge,.
While the temperature of the battery increases when the battery is charged, the rate the temperature rises(dT/dt) increases when the battery is nearing the full charge. The second conventional method, which utilizes this phenomenon, detects the full charge based on the rate the temperature rises.
In the following, the second conventional method will be explained.
Because the temperature rise may cause the deterioration of the battery or may shorten the life of the battery, the battery generally be cooled. To cool the battery, one method (air cooling) uses air outside or inside the vehicle as the coolant by appropriately positioning the battery in the vehicle. Another method (water cooling) uses a water jacket and a heat exchanger (radiator) with a specific coolant water. The temperature of the battery is adjusted by one of these methods. The second conventional method detects the rate the temperature rises when the temperature is controlled.
While the hybrid vehicle preferably uses an external charger which can constantly output a stable voltage when charging the battery, the external charger increases the cost of the vehicle.
Therefore, the battery should be charged by the generator rotated by the engine. To fully charge the battery, the engine is idled to actuate the generator, from which the electric power is supplied to the battery. However, the engine rotational speed may be irregular and unstable, and therefore the electric power produced thereby may fluctuate. Further, the electric power, which is generated by the generator rotated by the engine, may be consumed by other electric loads (e.g., a cooling fan, or an air conditioner). When these devices are operated during the charging of the battery, the charging voltage or current may fluctuate. Therefore, by the above-mentioned first conventional method, accurate detection of the full charge is impossible.
When cooling the battery, air outside or inside the vehicle may be used as the coolant. This method, however, must take into consideration water resistance and mud spatter, and this increases the number of parts for enhancing the cooling effect of the air conditioner. The air outside the vehicle is rarely used as the coolant because the packaging of the vehicle may be degraded. Mainly, the coolant is the air inside the vehicle. In this case, when the air conditioner may be started or stopped during the charging of the battery, or when the openings (e.g. the doors) of the carriage are opened and closed, the temperature of the air inside the vehicle may vary, affecting the temperature of the battery, which then may also vary. Therefore, by the above-mentioned second conventional method, accurate detection of the full charge is impossible.